TIP3P coordinate file

From wiki.gromacs.org

When using genbox to generate a box of solvent, you need to supply a pre-equilibrated box of a suitable solvent for genbox to stack around your solute(s), and then to truncate to give the simulation volume you desire. When using any 3-point model (e.g. SPC, SPC/E or TIP3P) you should specify -cs spc216.gro which will take this file from the gromacs/share/top directory. You should then be sure to equilibrate for at least 5-10ps at the desired temperature. You will need to select the right water model in your .top file, either with the -water flag to pdb2gmx, or by editing your .top file appropriately by hand.